This patent application claims priority based on a Japanese patent application, H11-153849 filed on Jun. 1, 1999, the contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a correcting method of an optical signal transmission system and an optical signal transmission system using said correcting method. More particularly, the present invention relates to a correcting method of an optical signal transmission system including photo-electric conversion, and an optical signal transmission system using said correcting method.
2. Description of the Related Art
An optical signal transmission system generally excels in reliability and transmission speed and rapidly broadens the fields of application, as a means of signal transmission. An optical signal transmission system, which utilizes an optical fiber, is sometimes used for transmitting a signal between a main body unit and a test head, on which a semiconductor device is mounted, of a semiconductor device testing apparatus. With recent rapid improvements of the performance of semiconductors, an apparatus that tests such devices has to be able to be operated with extremely high speed and high reliability.
One of the basic principles of an operation of an optical signal transmission system is: converting an electrical signal to an optical signal and applying this optical signal to an input end of the optical fiber, and converting an optical signal arising at an output end of the optical fiber into an electrical signal by a photo-electric conversion. An electric current driving type laser diode is mainly used for generating a light-emitting action at the input side of the optical fiber. A problem arises because there is unevenness in the relationship between the electric current that drives the laser diode and the brightness of a light of the laser diode driven by the electric current.
FIG. 1 shows a correlation between a driving current of the laser diode and brightness. Here, the characteristic of the diode is shown for three kinds of surrounding temperatures T=T0, T1, and T2. As shown in the figure, the laser diode does not oscillate when an electric current is below some value, and increases brightness linearly when an electric current exceeds some value. The value is called a threshold value, and the threshold value increases with the increase of the surrounding temperature. Also, there is unevenness of the threshold current among each of the laser diodes.
Because of the influence shown above, for example, if transmitting a high-speed clock, a gap may be caused between the threshold value of a signal expected at the light-emitting side and the light receiving side, so that a duty ratio of a clock may be changed, or a skew is caused among many signals which has to essentially be changed simultaneously. The phenomenon shown above becomes an obstruction to the increase of speed of transmission. Especially, for a semiconductor device testing apparatus, which is required to operate in a broad range from a direct current to a high frequency, it is difficult to solve the above problems with respect to both increasing the speed of operation and also maintaining stability of operation for each frequency.
Therefore,it is an object of the present invention to provide a series of technologies for realizing a preferable form of transmitting a signal using an optical signal transmission system, which overcomes the above issues in the related art. More particularly, it is an object of the present invention to provide correction technology that can perform a desired adjustment on a signal, which is to be transmitted by the optical signal transmission system, and the optical signal transmission system utilizing the correction technology. This object is achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
According to the first aspect of the present invention, a correcting method of an optical signal transmission system that applies an optical signal generated by light-emitting action to an input end of an optical fiber and converts an optical signal arising at an output end of the optical fiber to an electrical signal by photo-electric conversion can be provided. The correcting method includes: transmitting the optical signal by inputting the optical signal to the optical fiber and generating the electrical signal; and adjusting at least one of an electric current related to the light-emitting action and an electric current related to the photo-electric conversion according to an electric current of the electric signal.
The adjustment of an electric current may include: adjusting magnitude of an electric current, which generates the light-emitting action, so that the magnitude of an electric current of the electric signal and the magnitude of a predetermined reference current matches. The adjustment of an electric current may include: determining the reference current as two kinds of large and small values one after another; adjusting each of two kinds of values of electric currents that generate the light-emitting action according to the two kinds of large and small values; and holding each of the adjusted two kinds of values of electric current individually.
A small value from among the two kinds of large and small values may be determined by assuming a condition in which the optical signal has a faint intensity, which is not xe2x80x9c0xe2x80x9d. The correcting method may further include: judging whether the magnitude of an electric current, which generates the light-emitting action adjusted by the adjusting of an electric current, is within a predetermined permission level. The adjusting of an electric current may include: adjusting the magnitude of a reference current, which is used for detecting a magnitude of an electric current of the electric signal, at a circuit related to the photo-electric conversion so that the magnitude of an electric current of the electric signal generated at the transmitting of the optical signal and the magnitude of the reference current match.
The adjusting of an electric current may include: generating two values, which are to be shown by the optical signal sequentially; adjusting each of two kinds of values of the reference current; and holding the each of adjusted two kinds of values individually. The adjusting of an electric current may include: generating an intermediate value between the adjusted two kinds of values; and judging which of the two kinds of values is shown by the optical signal based on a comparison between the intermediate value and the magnitude of an electric current of the electric signal.
The adjusting of an electric current may include: adjusting a value of the reference current by generating one of two values to be shown by the optical signal; and holding the adjusted value of the reference current. The adjusting of an electric current includes: setting a value of an electric current, which is to be compared to the magnitude of an electric current of the electric signal, to judge which of the two values is shown by the optical signal, based on the adjusted value of the reference current.
The correcting method may further comprise: judging whether the adjusted two kinds of values adjusted at the adjustment of an electric signal is within a predetermined permission level. The correcting method may further comprise: judging whether the adjusted value of the reference current adjusted at the adjustment of an electric signal is within a predetermined permission level.
According to the second aspect of the present invention, an optical signal transmission system having a pre-processing circuit that includes a light-emitting circuit and processes a signal to be input to an optical fiber, and a post-processing circuit that includes photo-electric conversion circuit and converts a signal output from the optical fiber to an electric signal can be provided such that, the optical signal transmission system comprises: a current controlling circuit which adjusts an electric current of the pre-processing circuit or the post-processing circuit according to an electric current of the electric signal.
The current controlling circuit may adjust an electric current which generates a light-emitting action at the light-emitting circuit according to the electric current of the electric signal. The current controlling circuit may have a storing circuit which holds a magnitude of an electric current of the electric signal when the magnitude of an electric current of the electric signal matches a magnitude of a predetermined reference current. The storing circuit may include a circuit that holds the magnitude of an electric current of two kinds of the electric signal, each corresponding to each of the magnitude of the reference current having two kinds of large and small values.
The post-processing circuit may have a comparison circuit that compares a magnitude of the electric current of the electric signal and a magnitude of a predetermined reference current; and the current controlling circuit may include: a circuit that changes a magnitude of an electric current, which generates the light-emitting action, monotonously; and a circuit that fixes a magnitude of an electric current which generates the light-emitting action when a relationship between the magnitude of an electric current of the electric signal and the magnitude of the reference current reverses.
The circuit that changes the magnitude of the electric current monotonously may include: a counter circuit that performs increment operation or decrement operation; and a circuit that fixes the magnitude of the electric current including a masking circuit that stops the increment operation or decrement operation of the counter circuit. The optical signal transmission system may further include a circuit that judges whether the magnitude of the electric current which generates the light-emitting action adjusted by the current controlling circuit is within a predetermined permission level.
The current controlling circuit may comprise: a measuring circuit which measures a magnitude of the electric current of the electric signal; and a reference value generating circuit that sets a reference current for determining the electric signal in two values based on the measured magnitude of the electric current of the electric signal. The optical signal transmission system may further comprise an output circuit which determines the electric signal in two values based on the reference current. The measuring circuit may measure the magnitude of an electric current of the electric signal for each of two values, which is to be shown by the electric signal, individually; and the reference value generating circuit generates an electric current, the magnitude of which takes an intermediate value of the magnitude of an electric current of the electric signal measured individually, as the reference current.
The measuring circuit may measure the magnitude of the electric current of the electric signal for one of two values, which is to be shown by the electric signal; and the reference value generating circuit may set a value of an electric current to be compared to the magnitude of the electric current of the electric signal for judging which of the two values will be shown by the electric signal based on the measured magnitude of the electric current of the electric signal. The optical signal transmission system may further include a circuit that judges whether the magnitude of the electric current of the electric signal measured by the measuring circuit is within a predetermined permission level.
This summary of the invention does not necessarily describe all necessary features of the present invention. The present invention may also be a sub-combination of the above described features. The above and other features and advantages of the present invention will become more apparent from the following description of embodiments taken in conjunction with the accompanying drawings.